Polyester resins are polycondensation products of a dicarboxylic acid with dihydroxy alcohols. Such polyester resins catalyze to cure or harden, generally at room temperatures, to make a wide variety of products. Unsaturated polyester resins are widely used today to make products for use in the marine, transportation and building industries, and in particular, fiber or particulate reinforced products, such as glass fiber reinforced laminate-type products. Unsaturated polyester resins generally contain ethylenic unsaturation introduced by the employment of unsaturated diacids, thus comprising thermosetting-type resins. As such, unsaturated resins generally employ maleic and fumaric unsaturated acids, although saturated acids, phthalic and adipic acid may also be included to control or reduce the amount of unsaturation in the final unsaturated polyester resin and to control the physical properties. Dihydroxy alcohols most generally used are typically ethylene, propylene, diethylene and dipropylene glycols. Crosslinking agents may be employed with the unsaturated polyester resins. Generally, styrene monomers and di-allyl phthalate are the most common crosslinking agents. The styrene monomers are used to reduce the viscosity of the polyester resin and to act also as a crosslinker in the polyester resins.
It is desirable, particularly in view of the toxicity of the styrene monomer and government regulations, to reduce the concentration levels of the styrene monomer from the usual 40% to 50% by weight of the polyester resin to below 35% or 30% by weight. However, reductions in styrene monomer concentrations present problems in the polyester resins with the increased viscosity of the resin, causing difficulties in applying the resins, such as causing spray problems and glass roll-out when the resin is sprayed or used in conjunction with glass fibers. The reduced styrene monomer polyester resin does not wet the glass in preparing glass fiber laminates, and therefore, the surface of the sprayed glass fiber polyester resin typically must be contacted with a roller to prevent glass fibers from sticking out of the surface. In addition, the physical properties of the styrene monomer-reduced polyester resins are also greatly reduced without the use of additional, supplemental crosslinkers in the polyester resin. The aromatic nature of styrene monomers in the unsaturated polyols improves the hydrolytic stability and increases the chemical resistance to water, detergents and corrosion in the final, cured polyester resin product.
Acrylic monomers, such as methyl methacrylate (MMA) have been added to low or reduced styrene monomer polyester resins for marine grade gel coats and for outdoor applications. However, and unfortunately, MMA has a strong odor, is a skin and eye irritant and results in a slowing of the cure rate of a polyester resin. However, such acrylic monomers usually improve UV, water and chemical resistance. Therefore, it is not desirable to substitute MMA monomer for the styrene monomer or to add it with the styrene monomer, since MMA is also a hazardous material.
It is therefore desirable to provide new and improved, photoinitiated, crosslinkable resin compositions to replace or reduce styrene monomer in unsaturated resins, to provide unsaturated, cured resins with better physical and chemical properties and to provide an unsaturated polyester resin composition having reduced styrene monomer therein.